Proportional regulation in a fluid system is the action of a mechanism to vary fluid output pressure relative to the fluid input pressure in response to one or more varying control factors. The output pressure is generally controlled to effect a desired response from a fluid actuated element. This type of a control mechanism has useful application in the control of automotive braking systems.
It is typical for an automotive braking system to operate in a traditional base brake mode wherein manual actuation of a master cylinder effects a desired application of the wheel brakes. In addition to the base brake mode, braking systems are often capable of controlling vehicle deceleration through anti-lock operation, controlling vehicle acceleration through traction control operation and improving lateral and longitudinal vehicle stability through stability enhancement systems which provide a level of dynamic handling augmentation. Such multi-functional brake systems are becoming increasingly more common and therefore, providing an effective and economical multi-functional system is desirable.
Brake apply system designs are known wherein the pressure applied to a vehicle's wheel brakes is controlled by an electronic unit that evaluates several parameters and delivers a control signal to a hydraulic modulator that sets the wheel brake pressure. A key parameter used to determine the appropriate braking pressure is the driver's command, delivered as an input on the brake pedal. Braking systems that provide several distinct operating modes require a mechanism to "modulate" the braking pressure at the wheel brakes based on parameters other than, or in addition to, the driver's application of force to the brake pedal. A modulator typically includes a pressure generation mechanism and a means of controlling delivery of the generated pressure to the wheel brakes. This may take the form of a pump and proportional hydraulic valve, a pump with a pair of two way valves or a movable-piston variable pressure chamber device. The number and arrangement of these elements included in a braking system is determined by the system layout and selected control scheme.
There are many operating conditions to consider in designing a multi-functional braking system. During braking operation on a uniform road surface for a vehicle moving in a substantially straight line, the friction characteristic at the tire to road interface is similar for all four wheels. If the brakes are applied to slow the vehicle, it is preferable for the application rate to be consistent between the left and the right sides of the vehicle, to inhibit the introduction of brake induced yaw. If brakes are applied according to an automatic control mechanism for target path correction of the vehicle in maneuvering situations, then the application rates are selected to purposely introduce a yaw moment. Additionally, anti-lock and traction control braking operation often varies braking pressure between the individual wheel brakes of a vehicle. Therefore, it is preferable to have the ability to provide consistent braking pressure across the sides of a vehicle and also to have the ability to vary the braking pressure across the sides of a vehicle. The operating conditions are complicated by friction coefficient variances between wheels of the vehicle and other operational conditions.